Maximum session time

A feature request which I built for a project: a maximum session length with forced logout.

I've made a patch to accomodate for this feature request. It does quite a few things:
* Adds maximum session timeout fields (global and per role)
* I've changed minimum value (a static value of 60) to a constant
* The block caching disabled
* Use the calculated remaining time instead of the fixed length in various places
* A bit of code-cleanup (removed double spaces, unused variables, etc)
* The ahah-callback set_last now also sends the remaining time back
* Extra dialog when your session has hit the max-session length

(Patch in first comment)